This invention relates to an improved transistor type pulse welding device which utilizes a pulse current produced by a transistor.
A pulse welding device having a reactor assembly as shown in FIG. 1 and a circuit as shown in FIG. 3 is known in the art. In FIG. 1, reference numeral 1 designates a reactor coil for pulse current (hereinafter referred to as "an L.sub.P "), and 2 a reactor coil for background current (hereinafter referred to as "an L.sub.B "). These reactor coils 1 and 2 are wound on one and the same iron core (11 or 12). In FIG. 3, reference numerals 1 and 2 designate the aforementioned L.sub.P and L.sub.B, respectively; 3, a DC voltage source; 4, a pulse current switching transistor (hereinafter referred to as "a TR.sub.P "); 5, a background current switching transistor (hereinafter referred to as "a TR.sub.B "); 6, a circulation diode for pulse current (hereinafter referred to as "a D.sub.P "); 7, a circulation diode for background current (hereinafter referred to as "a D.sub.B "); and 8, an output terminal.
The operation of the conventional pulse welding device will be described. When only the transistor TR.sub.B is rendered conductive (on), an output current at output terminals 8 is increased while the rate of increase (di/dt) of the output current is being suppressed by the coil L.sub.B. When the transistor TR.sub.B is rendered non-conductive (off), a circulation circuit occurs in a path including the diode D.sub.B, and accordingly the output current is decreased. When the transistor TR.sub.P is rendered conductive, the output current is increased while the rate of increase is being suppressed by the coil L.sub.P. When the transistor TR.sub.P is rendered non-conductive, a circulation circuit occurs in a path including the diode D.sub.P, and accordingly the output current is decreased. Thus, an output current waveform as shown in FIG. 5 is obtained by controlling the on-off operations of the transistors TR.sub.B and TR.sub.P.
This operation will be described in more detail. When the transistor TR.sub.P is rendered conductive, the current starts to flow to the output terminal through the coil L.sub.P. However, since the coils L.sub.P and L.sub.B are wound on one and the same iron core as shown in FIG. 1, an induction voltage proportional to the rate of increase (di/dt) of the current flowing in the coil L.sub.P is induced in the coil L.sub.B. Since the induction voltage acts as a voltage source, a circulation current as indicated by the broken line in FIG. 3 tends to flow. This circulation current flows when the collector and emitter of the transistor TR.sub.B exhibit a diode characteristic. Sometimes, if the circulation current is several times as large as the background current, the transistor TR.sub.B will be damaged. If the collector and emitter of the transistor TR.sub.B does not exhibit a diode characteristic, a reverse voltage will be applied across the collector and the emitter. In this case, also, the transistor TR.sub.B may be damaged.
This circulation current problem is caused by the fact that the coils L.sub.P and L.sub.B are wound on one and the same iron core, so that the transistor TR.sub.B is damaged by an excessively large current flowing therein or by a reverse voltage applied thereto.
Furthermore, the circuitry shown in FIG. 3 is disadvantageous in that the switching frequency of the transistor TR.sub.B is usually high, which results in a large switching loss. It is therefore necessary to use a large capacity transistor and to employed an expensive cooling unit.